Seismic survey is a method for understanding a subsurface structure by artificially generating an elastic wave on an earth's surface or under water and analyzing a propagation behavior of the generated elastic wave. However, when a stratum has an inclination or a complicated geological structure, the elastic wave may be distorted or diffracted at a reflection site. Therefore, for the accurate subsurface imaging using seismic data, the migration that is a process of relocating a distorted reflection event to an accurate reflection position is required.
For the migration, Kirchhoff migration based on ray tracing and reverse-time migration using wave equations have been mainly used. In particular, the reverse-time migration using two-way wave equations may obtain high-accuracy images even in complicated geological features, but since the reverse-time migration using two-way wave equations requires much computation and relatively long operation time, the reverse-time migration has been mainly performed under the assumption that subsurface media is acoustic media having a few variables to be considered.
However, as apparatuses having computability like a supercomputer comes into wide use in recent years, the reverse-time migration in elastic media by which the subsurface structure may be reflected to be closer to reality has been performed. Unlike the acoustic media, a P-wave, an S-wave, a PS-converted wave, and so on are propagated through the elastic media. Here, each wave acts as noise for the other, and therefore anomalies different from the real subsurface structure may occur, such that the subsurface structure may not be accurately imaged.
Therefore, the reverse-time migration needs a process of separating each wavefield from the mixed wavefields. In connection with this, many studies on decomposing of the P-wavefield that may be propagated up to a deep depth have been conducted. For this purpose, the divergence operator has been applied to P-wavefield decomposition.
Thereafter, studies are being focused on anisotropy and three-dimensional data processing. However, the method for separating wavefields using a divergence operator requires a spatial differentiation for a stress component, such that waveform changing of the wavefield may be induced. Further, studies for clearly interpreting a subsurface stratum structure are necessarily required. In particular, notwithstanding that a subsurface structure of a high-velocity layer like salt dome that is a representative form of a reservoir in which petroleum and gas resources reserve is an important factor dominating a distribution form and a size of the reservoir, the general reverse-time migration method has a limitation of representing the subsurface structure. Therefore, a method that can overcome the problems is required.